The present invention relates to a device for positioning several units that can be moved adjacent to and at a constant distance from one another, each unit having a distance-traveled detector associated with its drive mechanism and the device as a whole having central controls that emit a reference value.
Positioning devices of this type are used in conjunction with machines that cut webs of material longitudinally to position units that have circular knives mounted on them. The units in a known positioning device are individually connected to the controls. The distance-traveled detector supplies the controls with the distance actually traveled. A value for that distance is then compared with a reference value representing the prescribed starting position for each unit and, in the event of a deviation, an appropriate readjustment signal is released to the mechanism that drives the unit. The distance-traveled detectors employed in positioning devices of this type supply counting pulses. Since the pulses generated by the pulse generator often get lost on the way to the counter, this method of measuring distance traveled is not especially reliable.
Contemporary angle encoders on the other hand supply absolutely reliable information as to distance covered and instantaneous position. Angle encoders consist of several parallel perforated disks that are driven by the same mechanism that drives the unit but at a different transmission ratio. Digital words that can be unambiguously associated with the positions of the moving units can be constructed from the various perforated disks.
An angle encoder of this type is preferably employed to determine the position of the units in the positioning device in accordance with the invention.